This invention relates to optical communications systems, and in particular to a device for diverting light among various optical transmission elements.
With the growth of optical communications systems, the need for coupling devices of various forms has arisen. In such systems, three and four port couplers could serve a variety of purposes. For example, in data communication transmission, bidirectional lines could be set up with the use of an efficient three port coupler where the bidirectional transmission line was at one port and the transmitter and receiver were coupled to a different one of the other ports. In systems where several terminals share a common central processing unit, it is desirable to provide a coupler which will permit each terminal to receive a portion of the light signal while allowing the bulk of the signal to be transmitted to other terminals. At the same time, the terminal should be able to transmit without interfering with its own receiver. Thus, an asymmetric four port coupler is needed. A further type of four port coupler, which can be thought of as a 1.times.3 power divider, may be useful in cable TV systems where it is desired that each station have the capability of transmitting back to the source of the signal. Thus, in such applications, a coupler is needed which will allow receiving a portion of the incoming signal at one port and transmitting a signal at another port in a direction opposed to the incoming signal while not interfering with the receiver at the other port.
Some proposals have been made for providing three and four port couplers. For example, U.S. Pat. No. 4,213,677 issued to Sugimoto suggests use of a beam splitter between two graded index of refraction lenses. Light from a fiber coupled to one surface of a lens is partially reflected by the beam splitter back to a fiber coupled to the same surface and partially transmitted to a fiber on the opposite surface of the other lens. Changing the reflectivity of the mirror adjusts the splitting ratio of the incoming light between the two output ports. While such a device performs acceptably, it is generally preferable to keep lens components to a minimum. Other types of three port couplers utilizing a single lens element have required some polishing or grinding which can cause fabrication difficulties.
A type of multiport coupler has been suggested in U.S. Pat. No. 4,304,460 issued to Tanaka. There, an array of fibers is coupled to one surface of a graded index of refraction lens and a rotatably mounted mirror positioned at an angle to the lens is provided at the other surface. Incoming light from one fiber is diverted to a selected one of the other fibers by a proper rotation of the mirror. Some multiplexing may also be accomplished by providing a filter element between the lens and the mirror. In most applications, it is believed that use of a rotating element adds undue complexity and cost to the coupler.
A particular design for an asymmetric four port coupler has been disclosed in U.S. patent application of Levinson and Schweizer, Ser. No. 373,135, filed Apr. 29, 1982 and assigned to the present assignee. There, at least partially reflecting elements are positioned between two graded index of refraction lenses and at an angle to each other. Two fibers on one surface of the lens opposite the reflecting elements provide the input and receiver ports, and two fibers on the opposite surface of the other lens provide the transmitter and output ports. The angle between the reflecting elements maximizes coupling between the input and output ports while isolating the transmitter and receiver ports. While such a device is generally advantageous, it is believed that use of a coupler with only one lens element for providing this function would cut down on cost and losses in optical transmission.
It is therefore an object of the invention to provide optical coupling devices which are economical and provide efficient coupling among transmission elements. It is a further object of the invention to provide such couplers where the ratio of light incident on the various elements may be conveniently altered to suit particular needs. Finally, it is an object of the invention to provide specific devices for three port coupling, asymmetric four port coupling and four port power dividing.